Oxygen separation



Patented Sept. 5, 1950 OXYGEN SEPARATION Henry J. Ogorzaly, Summit, N. J., assignor to Standard Oil Development Company. a corporation of Delaware Application June 12, 1946, Serial No. 676,237

5 Claims. (Cl. 62--175.5)

The present invention is concerned with an improved process for the manufacture of oxygen and nitrogen from air. The invention is more particularly directed to the fractionation stage, in which a two-tower fractionation sytem is normally employed. In accordance with the present invention carbon dioxide snow is removed from the cold air feed to the high pressure tower by recirculating the liquid bottoms through the lower part of the tower and passing a portion of the scrubbing stream through a filter or equivalent means to remove undesirable carbon dioxide.

snow.

It is wellknown in the art to manufacture oxygen and nitrogen from liquid air by fractionating the same at low temperatures. In these processes the air is cooled to liquefaction temperatures by various means and processes and then introduced to various fractionating equipment or means. A conventional method for example employs two fractionating towers, one a high pressure tower, and the other a low pressure tower. In an air separation process of high efficiency, the air is chilled to about -275 F. and introduced into the bottom of a high pressure tower. The pressure maintained in this tower is usually in the range from about 4.5 to 5.0 atmospheres, gauge. This high pressure tower com prises suitable fractionating plates. Substantially pure nitrogen is removed as an overhead vapor stream from the high pressure tower while the bottom stream comprises an oxygen-rich liquid stream containing from about 40 to 50% of oxygen. This oxygen-rich liquid is introduced at an intermediate point into a low pressure tower, which is operated at a pressure of about a few pounds per square inch above atmospheric pressure. The low pressure tower likewise contains fractionation means, generally bell cap trays. A vapor stream comprising substantially 100% pure nitrogen is withdrawn overhead from the low pressure tower and a substantially pure oxygen stream is withdrawn as a vapor from apoint near the bottom of the low pressure tower.

The heat required in the low pressure tower to vaporize the liquid feed to the column and also to vaporize the liquid reflux required for effective separation of the nitrogen and oxygen in this tower, is provided by condensing nitrogen vapor fromthe top of the high pressure tower. Due to the difference in pressure between the two fractionating columns, the normal temperature differential is reversed and the high pressure nitrogen condenses at a higher temperature than that required to boil the low pressure oxygen. Accordingly, a combination condenser-reboiler is generally employed, in which high pressure nitrogen vapors are condensed in tubes immersed in a pool of boiling liquid oxygen in the bottom of the low pressure tower. A portion of the condensed nitrogen is returned to the high pressure tower as liquid reflux, where it provides the liquid wash required for effective fractionation and also supplies the cooling required to produce a liquid bottoms from the vapor feed.

Another portion of the condensed nitrogen is introduced into the top of the low pressure tower to provide the liquid nitrogen wash required to free the ascending vapors of oxygen. It will be understood that with processes of low efliciency, or when liquid products are desired, the air inlet temperature and pressure may differ appreciably from those specified. Also, the design of the frac tionating system may differ, as, for example, a single fractionating tower may be used at a sacrifice in the efiiciency of air separation.

It is a common characteristic, however, of all air separation processes depending on the fractionation of liquefied air, that the final separation of gaseous oxygen is made at a temperature far below the solidification point of carbon dioxide. Also at these temperatures the vapor pressure of carbon dioxide is so low that only a very small proportion can be supported in the vapor state and vented from the unit in the gaseous oxygen product. For example, in a two-column process operating with a pressure of about 5 pounds per square inch, gauge in the oxygen reboiler and eflecting substantially complete separation of air into gaseous oxygen and nitrogen, only 0.001% of the carbon dioxide in the inlet air can be vented from the fractionating system in the product streams. As a consequence, if removal of carbon dioxide during preliminary stages, as by chemical treatment or freezing out, is not extraordinarily complete, solid carbon dioxide accumulates in the fractionation system, principally in the oxygen reboiler. A decrease in heat transfer efliciency and eventually an interruption of the operation accordingly results. I have now discovered a process by which carbon dioxide snow in the chilled gaseous air feed to the fractionating system can be readily removed in an economical manner resulting in continuous operation at increased efficiency.

My invention is de cribed with reference .to a two-tower fractionating system. In accordance with my invention I extend the bottom of the high pressure tower, or by expressing it in another way I withdraw the feed stream to the 10 pressure tower from a point somewhat above the bottom of the high pressure tower. I introduce the chilled gaseous air stream into the bottom of the tower and scrub this stream with a recirculating liquid stream in the lower portion of the tower. A part of this scrubbing stream, which normally contains about 40-60% oxygen, is passed through filtering means to remove the carbon dioxide snow and then added into the main stream from the high pressure tower, representing the feed to the low pressure tower.

The process of my invention may be readily understood by reference to the attached drawing illustrating one modification of the same.

In accordance with my process, I provide a high pressure tower l sufllciently elongatedor upwardly extending to provide a scrubbing section. The feed air stream is introduced at the bottom of the scrubbing section by means of line 20. The high pressure tower is operated under conditions so adjusted as to temperature and pressure that there is removed overhead through line 3 a stream which is substantially 100% nitrogen. An oxygen-rich liquid feed to the low pressure tower is withdrawn at a point above the scrubbing section by means of line 4. This oxygen rich stream generally contains from about 40-60% oxygen. This stream is introduced through a throttling valve ll into an intermediate point in low pressure tower 5, as indicated in the drawing, and the tower is operated under conditions adapted to cause removal overhead of a stream in line B which is substantially 100% pure nitrogen. The liquid fed flows downwardly in low pressure tower 5 through the stripping section in which it is freed of nitrogen by contact with vapors rising from a reboiler 12. A nitrogenfree liquid stream is vaporized in the reboiler I2 and a part of the vapor passes upwardly in the tower, while another part of the vapor is withdrawn as oxygen product from low pressure tower 5 through line 1 as shown in the drawing. The heat supplied to the bottom of low pressure tower 5 is maintained by condensing the nitrogen leaving tower i via line 3. A part of the condensed nitrogen is removed by means of line M to provide reflux in high pressure tower i while another partis introduced into the top of low pressure tower 5 through throttling valve I3. This liquid reflux stream scrubs out the oxygen from the vapors ascending in tower 5 and a substantially oxygen-free gaseous product is delivered through line 6. A small gaseous vent stream is released from the top of the nitrogen condenser 9, through line I 5 in order to purge non-condensible permanent gases which would otherwise reduce the heat transfer. Certain heat exchange units in which the liquid streams in lines 4 and H] are normally sub-cooled by the gaseous stream in line 6 for greatest efficiency are not shown for purposes of simplification, and because they are not germane to my invention.

My process, as described up to this point, dilfers from prior practice in that the oxygen-rich liquid fed to the low pressure tower is withdrawn at a point above the scrubbing section and furthermore, there is withdrawn from the bottom of the tower by means of line 2| a liquid stream which is recycled by means of pump 25 to the top of the scrubbing section so as to countercurrently contact and wash the upfiowing gases free of suspended solid carbon dioxide. A portion of this stream is passed through filtering means 22 to remove therefrom the solidified carbon dioxide snow and then combined with the main stream free of carbon dioxide snow which is withdrawn from tower I by means of line 5. By operating in this manner, I am able to substantially completely remove the solidified carbon dioxide snow present in the feed gas introduced into high pressure tower l in an economical manner.

Filter 22 may comprise any suitable filtering means. However a bed of solids in a state of relatively fine subdivision is preferred, as, 1'01 example, 0-60 mesh sand or commercially available filtering earths. Filter 22 is preferably supplied in duplicate, so that one unit may be purged of retained CO2 snow, while the other continues in service.

The process of my invention comprises employing a scrubbing section in the bottom of the high pressure tower and withdrawing an oxygen-rich liquid stream from a point immediately above this scrubbing section for feed to the low pressure tower. From the bottom of the scrubbing section, I withdraw an oxygen-rich liquid stream and recirculate this stream to the top of the scrubbing section. The gaseous feed stream of air containing suspended particles of carbon dioxide snow is introduced into the bottom of the scrubbing section and thus is countercurrently washed free of carbon dioxide snow by the downfiowing liquid recirculating phase. A portion of this counter-.

This oxygen-rich liquid stream freed of carbon dioxide snow is combined with the oxygen-rich liquid phase removed from the high pressure tower immediately above the scrubbing section, to form the total feed to the low pressure tower. Periodically, the filter is freed by purging of retained solid carbon dioxide and thus returned to a high state of effectiveness. The amount of oxygen-rich phase recycled to the top of the scrubbing section as compared to the amount passed through the filtering means may vary considerably. However, in general I prefer that approximately to 9 of the oxygen-rich phase withdrawn from the bottom of the scrubbing section be recycled to the top of the scrubbing section, while from 5% to 20% of the oxygen-rich phase withdrawn from the bottom of the scrubbing section be passed through the filtering means and combined with the oxygen-rich phase withdrawn from the bottom of the fractionating section.

It is also within the scope of my invention to operate the scrubbing section as a, vessel physically separated from the high pressure fractionating tower, serving to provide the fractionating tower with a carbon dioxide-free feed. Scrubbing would be accomplished byrecirculation of the scrubbing liquid, with makeup of oxygen-rich liquid supplied from the bottom of the high pressure fractionating tower as required to maintain liquid level. The carbon dioxide snow washed out into the recirculating liquid stream is removed by filtration of a portion of this stream. The filtrate may either be returned to the scrubbing vessel or added to the liquid bottoms product of the high pressure tower.

My invention is not limited to any particular temperature or pressure conditions maintained in the high pressure tower. Furthermore, it is not necessarily limited to a two-tower system. My invention may be applied in any process of the character described wherein carbon dioxide to a temperature at which carbon dioxide snow solidifies, theimprovement which comprises introducing the feed stream into a lower portion of a fractionating zone comprising an upper fractionating section and a lower scrubbing section,

withdrawingfrom a lower portion of said scrubbing section an oxygen-rich liquid phase and recirculating the same to the top of said scrubbing section, withdrawing overhead from the top of said tower a nitrogen-rich phase, withdrawing from the bottom of said fractionating section an oxygen-rich phase and removing at least a portion of the carbon dioxide snow from the oxygen rich phase withdrawn from the bottom of said scrubbing section.

2. Process in accordance with claim 1 in which the temperature of the air introduced into the bottom of the washing section is below about -260 F.

3. In a process for the production of oxygen and nitrogen from air in a system comprising a high pressure fractionating zone and a low pressure fractionating zone and wherein carbon dioxide snow is present in the feed gases, the improvement which comprises utilizing a high pressure zone, consisting of a high pressure fractionating section and a lower washing section. introducing the feed air into the bottom of said washing section, withdrawing from a lower portion of said washing section'an oxygen-rich liquid phase and recirculating at least a portion of said phase to the top of said washing section, withdrawing from the top of said high pressure zone a nitrogen-rich phase, withdrawing from a lower portion of said fractionating section an oxygenrich phase, removing at least a portion of the carbon dioxide snow contained in said last-named phase, then passing said oxygen-rich phase to a low pressure zone and segregating in said low pressure zone a phase comprising essentially 100% nitrogen and a phase comprising substantially nitrogen-free oxygen.

4. Process in accordance with claim 3 in which a portion of said oxygen-rich phase withdrawn from a lower portion of said washing section is filtered to remove suspended carbon dioxide snow and in which said phase, free of carbon dioxide snow, is combined with the oxygen-rich phase withdrawn from a lower portion of said fractionating section.

5. Process in accordance with claim 3 in which the phase withdrawn from a lower portion of said washing section is segregated into one portion comprising to which portion is recycled to the top of said washing section, and a second portion comprising 5 to 20% of the total, and in which said latter portion is filtered to remove the carbon dioxide snow, which phase, free of carbon dioxide snow, is then combined with the oxygen-rich phase withdrawn from a lower portion of the stripping section and passed to a low pressure tower, withdrawing nitrogen overhead and oxygen as a bottoms product from said low pressure tower.

HENRY J. OGORZALY.

No references cited. 

1. IN A PROCESS FOR THE MANUFACTURE OF OXYGEN AND NITROGEN FROM AIR WHEREIN THE AIR IS CHILLED TO A TEMPERATURE AT WHICH CARBON DIOXIDE SNOW SOLIDIFIES, THE IMPROVEMENT WHICH COMPRISES INTRODUCING THE FEED STREAM INTO A LOWER PORTION OF A FRACTIONATING ZONE COMPRISING AN UPPER FRACTIONATING SECTION AND A LOWER SCRUBBING SECTION, WITHDRAWING FROM A LOWER PORTION OF SAID SCRUBBING SECTION AN OXYGEN-RICH LIQUID PHASE AND RECIRCULATING THE SAME TO THE TOP OF SAID SCRUBBING SECTION, WITHDRAWING OVERHEAD FROM THE TOP OF SAID TOWER A NITROGEN-RICH PHASE, WITHDRAWING FROM THE BOTTOM OF SAID FRACTIONATING SECTION AN OXYGEN-RICH PHASE AND REMOVING AT LEAST A PORTION OF THE CARBON DIOXIDE SNOW FROM THE OXYGEN RICH PHASE WITHDRAWN FROM THE BOTTOM OF SAID SCRUBBING SECTION. 